Is it during a race aswell or is it just at the initial start from when you start from a stand still?

If it's just the start that it's happening in then your tune might be giving you problems. When your starting a race your pit man holds your car off the ground so you can rev it up and clean it out and then when he puts it down it might be fuelling up and causing you to bog down at the start and once you got the revs up and cleaned it out during the race it seems fine.

Or it could be your clutch. What set up are you using white or yellow shoe? What are your tolerances and pre load? And what motor is it?

PS: Sorry after reading your original post again i realised it's happening all race so personally i would go over my clutch again. I run a RB and i find i have to use more pre load on my clutch spring than what the manual says to get the punch i like